In recent years, blue and white light emitting diodes (hereinafter, light emitting diode is abbreviated as “LED”) each having high luminance have been developed, and have been finding use in an expanded variety of applications including bulletin boards, full-color displays, and backlights for portable phones. An acid anhydride curable epoxy resin has been conventionally used as an encapsulant for a photoelectric conversion element such as an LED because the resin is excellent in colorless and transparent nature.
An alicyclic acid anhydride such as methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, or tetrahydrophthalic anhydride has been generally used as a curing agent for an epoxy resin for use in such photoelectric conversion element.
However, when the alicyclic acid anhydride such as those mentioned above is used as a curing agent for an epoxy resin, a curing accelerator must be added in order that the alicyclic acid anhydride may be sufficiently cured because the alicyclic acid anhydride has low curing reactivity.
For example, triphenylethylphosphonium bromide, tetrabutylphosphonium bromide, (Patent Document 1), 2-ethyl-4-methylimidazole (Patent Document 2), a 2-ethyl hexanoate salt of 1,8-diazabicyclo(5,4,0)undecene-7 (Patent Document 3), or tetraphenylphosphonium bromide (Patent Document 4) has been used as a curing accelerator to be added in this case.
In order that an epoxy resin composition finds use in blue LED and white LED applications, an epoxy resin cured product is requested to be colorless and transparent under a long-term heating condition because the epoxy resin cured product is exposed to strong light emission energy of any such LED at a high temperature for a long time period.
When such curing accelerator as described above is used, the curing accelerator itself changes to a yellow color owing to long-term heating, and hence its colorless and transparent nature is apt to be impaired. Accordingly, the curing accelerator is not preferably used in an encapsulant for a photoelectric conversion element such as an LED.
In addition, curing agents each containing cyclohexane-1,2,4-tricarboxylic acid-1,2-anhydride (also called hydrogenated trimellitic anhydride and hereinafter abbreviated as “H-TMAn”) have been known as curing agents for obtaining colorless, transparent epoxy resin cured products each having improved light fastness and improved heat resistance (Patent Documents 5 to 8).
However, the epoxy resin compositions disclosed in those patent documents each have the following drawback. That is, each of the compositions has a high viscosity and a high curing rate, and hence shows a high degree of viscosity increase in standing at room temperature during a daytime operating time. Accordingly, each of the compositions is poor in workability.
Patent Document 1: JP 2000-344868 A
Patent Document 2: JP 2001-114868 A
Patent Document 3: JP 2002-97251 A
Patent Document 4: JP 2003-26763 A
Patent Document 5: JP 2005-36218 A
Patent Document 6: JP 2006-182961 A
Patent Document 7: JP 2007-39521 A
Patent Document 8: JP 2007-39522 A